It has recently been found, as evidenced by an article entitled "A New Physical Mechanism for Soft Errors in Dynamic Memories" by Timothy C. May and Murray H. Woods of Intel Corporation, Santa Clara, Calif. 95051, (a copy of which will be on file in the Patent and Trademark Office file for this invention) that a new physical soft error mechanism in dynamic random access memories (dynamic RAM's) and charge-couple devices (CCD's) is probably responsible for generating random errors in such memories by passage of ionized radiation through the memory array areas. It is suspected that alpha particle radiation can have sufficient energy to upset the state of particular memory bits within such memory devices.
A definition of "soft errors" is set forth as random, non-recurring, single bit errors in memory devices. Such errors are not permanent; that is, no physical defects are associated with the failed bit. In fact, a bit showing a soft error is completely recovered by the following WRITE cycle with no greater chance of showing an error than any other bit in the device. Since such errors are of a random nature and since they apparently are due to artificial sources such as polonium-210 and thorium-230, as well as natural sources such as uranium/thorium minerals which are present in the silicon materials forming such memory devices, they pose a problem for such memory devices.
In the past it has been a goal to reduce the incidence of such soft errors by such techniques as increasing the number of electrons necessary to differentiate between "empty" and "full" wells associated with such dynamic memories. In essence, such memories store a 0 or 1 by the respective emptiness or fullness of the associated bit well. It is known that by requiring more electrons to designate a "full" well, the chances of such soft errors are reduced. However, the more electrons necessary for designating a full well, the greater the physical size required for that particular data bit. This is obviously contrary to the goals of obtaining tighter memory packing densities. At present approximately 100,000 electrons are necessary for designating a "full" well in the highest density dynamic memory devices available. If the number of electrons is further reduced, the probability of soft errors dramatically increases as noted in the May et al article.
The present invention provides a unique and simple solution to this problem of soft errors whether they be with respect to dynamic RAM's or any other type of data memory device. Specifically, the present invention systematically examines each addressable memory location in the memory device and by use of standard error or Hamming codes determines the presence or absence of an error in that particular addressable memory location. Typically, such addressable memory locations use one or more data bytes; that is groups of 8 bits of data.
If an error is detected, the error correction code, via standard technology provides the necessary information to correct the soft error which in turn is then systematically rewritten into the memory location so that the memory location then stores the correct data. Since the review of each addressable memory location is performed on a systematic and relatively short time period basis (such as 1-10 seconds for the entire memory device), the chances of a multiple soft error occurring in any particular addressable memory location is virtually zero. Thus, the integrity of the memory is maintainable over an extended time period, and the fact that soft errors occur randomly throughout the memory device is completely irrelevent to the maintenance of the memory integrity.
Furthermore, use of the present invention's systematic memory detection and correction apparatus with dynamic memory devices, fewer electrons are necessary to designate a "full" well (a 1 state) than is presently feasible without usage of the present invention. Therefore, higher packing densities for such memory devices is a direct consequence of utilization of the present invention.